For many years the primary material used to control water hardness in detergent products has been sodium tripolyphosphate at levels of approximately 50% by weight of the finished detergent product. Within the past few years the use of high levels of sodium tripolyphosphate has come under scrutiny because of the suspicion that soluble phosphate species accelerate the eutrophication or aging process of water bodies. This eutrophication is ordinarily evidenced by the rapid growth of algae in the water body.
Sodium tripolyphosphate exists as a molecule containing 5 atoms of sodium, 3 of phosphorus, and 10 atoms of oxygen. When utilized as a detergent builder the sodium tripolyphosphate molecule sequesters as a soluble species one mole of calcium or magnesium cation per mole of tripolyphosphate anion. In other words, sodium tripolyphosphate sequesters calcium and magnesium ions on a 1:1 mole basis. The calcium or magnesium tripolyphosphate species is relatively stable in a wash solution, thus preventing the water hardness from reacting with anionic detergents which in turn gives better cleaning. The calcium or magnesium tripolyphosphate species exists essentially as a single trivalent negative anionic species in the wash solution. This calcium or magnesium trivalent anion does not precipitate in the course of the wash.
Sodium pyrophosphate has generally been considered to be an equivalent builder when compared to sodium tripolyphosphate. It has also been suspected that sodium pyrophosphate is the equivalent of sodium tripolyphosphate in promoting the eutrophication as has been discussed above.
Sodium pyrophosphate contains one less atom of phosphorus than does sodium tripolyphosphate. The foregoing is reflected in the empirical formula of sodium pyrophosphate which is 4 atoms of sodium, 2 atoms of phosphorus and 7 atoms of oxygen. Sodium pyrophosphate is also known to form a 1:1 molar complex with calcium or magnesium ions. The species resulting from the sequestration of calcium or magnesium by the tetravalent negative pyrophosphate anion is the calcium or magnesium divalent negatively charged pyrophosphate complex. This calcium or magnesium pyrophosphate complex is sufficiently stable in the wash solution to prevent the water hardness cations from interfering with the detergency process.
The pyrophosphate tetravalent anion has a high association constant with the first calcium ion with which it associates. Association constants are a measure of the stability of the complex formed. The divalent anion has a very small association constant with the second calcium ion for the formation of dicalcium pyrophosphate which is electrically neutral. In the absence of any material which makes the dicalcium pyrophosphate more stable, one of the associated calcium ions will be free to disassociate and to seek a more stable association such as with body soil on the fabrics or with the anionic detergent. The main purpose of controlling calcium ions whether free or associated in a weak complex is to prevent the last mentioned reaction with the detergent or soiled fabric from occurring. As the pyrophosphate anion strongly holds one mole of calcium ion per pyrophosphate anion it has been common practice to attempt the first association (sequestration) on a mole for mole basis. If, however, the pyrophosphate anion can be induced to strongly associate (precipitate) with two moles of calcium ion the pyrophosphate level used could be reduced substantially, and still maintain the same level of hardness control. Preferably some free pyrophosphate tetravalent anion will also be present in the wash for its value in peptizing clay soils.
Thus, in addition to the molecular weight advantage which allows more moles of the pyrophosphate salt to be present in a composition at a given fraction of phosphate than tripolyphosphate, the ability of the pyrophosphate to precipitate as the dicalcium pyrophosphate salt under normal wash conditions gives a substantially greater advantage. However, the precipitation of pyrophosphates has been viewed as undesirable in wash solutions as the salt formed has a tendency to build up on fabrics and exposed machine surfaces. Furthermore the precipitation of calcium pyrophosphate is unpredictable under normal wash conditions where such factors as the total hardness, the pH, and the ratio of calcium ions to magnesium ions may vary from load to load.
Some work has been done in an attempt to make pyrophosphate a more effective detergency builder. Johnson states in U.S. Pat. No. 2,381,960 issued Aug. 14, 1945 that water-hardness may be reduced by adding pyrophosphates to the solution containing the hardness after a supplemental alkaline material such as sodium orthophosphate, alkali metal hydroxides and carbonates, soap or sodium silicate having an SiO.sub.2 :Na.sub.2 O ratio greater than 1.5 has been added to the solution. Kepfer in U.S. Pat. No. 2,326,950 issued Aug. 17, 1943, discloses that pyrophosphates can be used to control water hardness if the pyrophosphate is added to the solution containing the hardness prior to the addition of certain specific supplemental alkaline materials such as sodium borate, sodium metasilicate and disodium dihydrogen phosphate. Kepfer, in effect, teaches exceptions to the general rule set forth by Johnson. It has been found that Johnson was wrong, at least in one respect, as detailed hereinafter.
It can thus be seen that pyrophosphate has the potential of being a much more effective detergency builder than do the tripolyphosphate salts.
It has now been discovered that alkali metal pyrophosphates can be formulated into a detergent composition in a manner such that the pyrophosphate builds by first associating with one mole of calcium and then precipitating up to two moles of calcium per mole of pyrophosphate. An advantage to the precipitation of pyrophosphate in large amounts is that the precipitate can be removed from waste water and used as a fertilizer in the sewerage sludge.
It is thus an object of the present invention to more efficiently utilize alkali metal pyrophosphates as detergent builders.
It is a further object of the present invention to provide a reduced phosphorus content detergent product without substantially impairing cleaning in hard water.
It is yet another object of the present invention to utilize an alkali metal pyrophosphate salt in such a manner that it precipitates as the dicalcium salt thereof.
It is yet a further object of the present invention to provide greater cleaning due to increased hardness control in a product of limited phosphate content.
It is yet a further object of the present invention to diminish the deposition of water hardness salts upon fabrics in the wash solution.
These, and other objects which will become apparent, are to be achieved by processing the alkali metal pyrophosphate and the detergent with a high ratio alkali metal silicate such that substantially all of the moisture is removed from the resultant product.
Percentages and ratios given throughout the application are by weight unless otherwise indicated. Temperatures are in degress Fahrenheit unless otherwise noted. The term dry weight basis indicates that the slurry when dried would have the same weight percentages in a finished product.